Vestibular A&P Flashcards
Inner ear/vestibulo-cochlear apparatus
The vestibule-cochlear apparatus is embedded in the temporal bone
How big is the inner ear?
~3-5 mm
Bony vs Membranous Labyrinth
Membrane is within body cavern (bony labyrinth) and filled with endolymph
Vestibular system functions
- equilibrium, organ of balance
- sense motion, position of head/body
- maintains head upright
- maintains balance and how we perceive space
- postural reflexes
- influencing eye movements
- monitors position, velocity, and acceleration
2 sets of organs that detect motion
Semicircular canals and otolith organs (utricle, saccule)
What movement is detected by semicircular canals?
Detects angular acceleration (yes, no, head tilt)
What movement is detected by the otolith organs?
Linear acceleration
- Utricle = horizontal plane (car, tipping)
- Saccule = vertical plane (elevator)
What are the semicircular canals?
3/ear: arranged perpendicular and complementary
- Superior (nodding)
- Posterior (head tilt)
- Horizontal (shake no)
What are the SSC complementary pairs?
“Push pull”
- 2 horizontal canals
- R anterior/L posterior
- L anterior/R posterior
…During movement, one ear signals motion by excitation, and inhibition of the other complementary canal.
Parts of the SSCs
Ampulla: small swell at base of canal (sensory)
Crista: elevated ridge in ampulla (like macula) where cilia emerge.
Cupula: gel mass where cilia are embedded. Covers hair cells.
How does the SSC detect angular acceleration?
During angular acceleration, fluid lags displacing the cupula/hair cells.
Ex: L head turn = inertia of endolymph pushes cupula, deflecting hair cells (toward kinocilium depolarize +, away from kinocilium hyperpolarize -).
What happens when the SSC is dysfunctional?
Vertigo, dizziness, nausea symptoms!
What are the parts of the otolith organs?
Macula: sensory epithelium of utricle and saccule
- consists of hair cells with cilia embedded in gel matrix
- gel has calcium carbonate crystals embedded (otoconia)
Otoconia: provides inertia, w movement, otoconia/gel drags the hair cells
What are the vestibular hair cells?
One kinocilium and many stereocilia in each hair cell. Towards kinocilium = depolarization/excitation. Away from kinocilium = hyperpolarization/inhibition.
Converted to a neural signal thru CNVIII
Utricle response to movement
Head tilt back: toward kinocilium +
Head tilt forward: away from kinocilium -
Acceleration: toward +
Deceleration: away from -
Activation of hair cells (otolith organ)
Movement of otolith membrane - hair cells bend toward kinocilium - K+ opens - depolarization
Movement stimulates both excitatory and inhibitory
Signal transmission: PNS to CNS
PNS
1. Electrical stimulus from periphery to Scarpa’s ganglion
2. CNVIII Vestibular nerve (superior/inferior)
CNS
3. Vestibular nuclei (sup/inf/med/lat)
4. Brainstem (VN) or cerebellum (flocculonodular lobe)
PNS to CNS Connections for Eye Movement
- SSC/Otolith organs
- Vestibular nerve
- Vestibular nuclei
- Medial longitudinal fasciculus
- Oculomotor nuclear complex
- Extra ocular muscles
What is the vestibular nuclei complex?
- Occupy large area in lateral brainstem just below floor of 4th ventricle
- Each have specific connections and functional characteristics
Vestibular Nuclei Complex - CNS Connections
- Autonomic nervous system
- Cerebellum <>
- MVST and LVST (VS reflexes)
- Cervical proprioceptors (head movements) <>
- Peripheral vestibular organs
- Ocular motor nuclei (vestibulo-ocular reflex)
- Cerebral cortex (perception)
What are the vestibular central connections?
- Cerebral cortex (perception)
- Spinal cord (postural reflexes)
- Cerebellum (VOR compensation, mvmt refinement)
- Reticular formation (reticulospinal tract)
- Oculomotor nuclei (VOR)
Vestibular connection - Spinal Cord
Vestibular nuclei give rise to important descending paths in SC-VST
- Lateral VST from lateral VN
- Posture, extensor mm. (thru lumbar spine) - Medial VST from medial VN
- Reflexive head/neck control (vestibulocolic reflex)
Vestibular connection - Cerebellum
Vestibular nuclei have 2 way communication with cerebellum
Input = head movement info to cerebellum
Output = cerebellum terminates on lateral VN - SC to regulate muscle tone and coordination
Output = cerebellum feeds back to brainstem to adjust/coordinate eye movements
Vestibular Connection - Reticular Formation
- Reciprocal connection between vestibulonuclei complex and reticular formation
- Reticulospinal tract descends to SC for locomotion and postural control
- Key role in triggering nausea/vomiting
Vestibular Connection - Oculomotor Nuclei
- Vestibular nuclei connect to CN III, IV, VI through MLF
- VOR: conjugate eye movement with vestibular stimulation
- Involve lateral/medial/superior vestibular nuclei
Vestibular Connection - Cortex
- Perception of body orientation in space via thalamocortical path
- Vestibular nuclei to thalamus to higher cortical centers for perception of movement/equilibrium
Connections enable vestibular system to regulate posture, eye and head movements
VOR
- gaze stabilization while head moves
- producing eye movements that counter head movements
- SSC directly controls eye (head R = eyes L)
- faster than visual tracking
VOR: R Head Turn
- R horizontal SSC (+) - L lateral rectus and R medial rectus
- L horizontal SSC (-) - inhibits L medial rectus and R lateral rectus
VOR Gain
- Ratio of eye movement / head movement
- Ideal = 1
- Peripheral vestibular issue = low gain
